This invention relates to a modulator for the S-band frequency of a transmitter capable of providing both linear phase modulation and quadriphase shift key (QPSK) modulation.
In a tracking and data relay satellite system (TDRSS), a user transponder included a staggered QPSK modulation as well as a linear phase modulation capability, with close tolerances on the amplitude and phase relationships of the four phase states of the QPSK modulator output and on the linearity of the linear phase modulator. The desired precision required that a carrier-phase-switching type of QPSK modulator be used, rather than any form of linear modulation. Furthermore switching between 0.degree. and 180.degree. was greatly preferred for the same reason over switching smaller steps with subsequent multiplication.
In the past, these requirements have been met by a QPSK modulator at the S-band transmitter output frequency and in series with a linear modulator. However, better reproducibility and more freedom from parasitic capacitance and inductance effects could be obtained from operating the QPSK modulator at a lower frequency while maintaining operation of the linear modulation at a higher frequency, typically one-eighth the transmitter S-band output frequency. The present invention makes possible the arrangement of QPSK and linear phase modulation with these desired characteristics and performance.